[c#, improvement]: add explicit namespaces to custom config

[dcb6]

The C# generator now supports configuration to match namespaces directly to the full path of a file.
This can lead to more imports being required to use the SDK, but can be helpful to avoid collisions.

- name: fernapi/fern-csharp-sdk
  version: 0.3.2
  config:
    explicit-namespaces: true

[dcb6]

example of diff here
previously would've been just

namespace SeedExhaustive;